1. Field of the Invention
The present invention relates to a piezoelectric resonator including a vibration unit having a multilayer structure including thin-films made of piezoelectric and dielectric materials, and more particularly, to a piezoelectric resonator which is used in a filter, an oscillator or other suitable apparatus and is operated so as to be thickness-longitudinally vibrated in a VHF band, a UHF band, and ultra-high frequency band of which the frequencies are higher than those of the these bands. Moreover, the present invention relates to a filter and an electronic communication device including the piezoelectric resonator.
2. Description of the Related Art
In some piezoelectric resonators using thickness-longitudinal vibration, a resonance response is achieved in an ultra-high frequency band by use of a structure in which a piezoelectric film with a very small film thickness is interposed between electrodes, utilizing an inversely proportional relationship between the resonance frequency and the thickness of the piezoelectric film.
Of the above-described piezoelectric resonators, some of the thickness-longitudinal vibration type are each provided with a substrate having a hole passing through the substrate from the front surface to the back surface, a diaphragm made of an SiO2 thin film disposed on the substrate so as to cover the hole, and a vibration unit including a ZnO thin film interposed between a pair of opposed electrodes on the diaphragm.
Some of the above-described type piezoelectric resonators have a structure in which a piezoelectric film is sandwiched between a pair of upper and lower electrodes, and in particular, and have a structure in which the loops of a fundamental wave is positioned on the electrodes to eliminate the loss of resonance energy caused on the electrodes, so that the resonance characteristics are improved.
The loss of the resonance energy is reduced since the fundamental wave is used. However, in use of the fundamental wave, the resonance frequency temperature coefficient is considerably changed, due to the variation of the ratio of the thicknesses of the SiO2 thin-film and the ZnO thin-film. Therefore, the resonance frequency tends to be significantly changed with the temperature. Thus, for the piezoelectric resonator, the stability of the resonance frequency to the change of temperature is low.